83 Per Cent of Radiologists Didn't Spot the Gorilla Hiding in This CT Scan

You've almost certainly seen the dancing gorilla video which demonstrates the theory of change blindness — a phenomenon which means we don't see changes we're not expecting. Now, an updated experiment shows that the same may be true of radiologists analysing CT images.

A team of psychological scientists from Brigham and Women's Hospital in Boston wanted to work out if the previous dancing gorilla experiment ony worked because test subjects were naive, untrained, and thus less aware. So they thought they'd see if radiologists — the physicians who analyse medical images like MRIs, X-rays and CT scans — would fall for a similar trick. The Association for Psychological Science explains:

They recruited 24 experienced and credentialed radiologists-and a comparable group of naïve volunteers. They tracked their eye movements as they examined five patients' CT scans, each made up of hundreds of images of lung tissue. Each case had about ten nodules [minuscule signs of lung cancer] hiding somewhere in the scans, and the radiologists were instructed to click on these nodules with a mouse. On the final case, the scientists inserted a tiny image of a gorilla (an homage to the original work) into the lung. They wanted to see if the radiologists, focused on the telltale nodules, would be blind to the easily detectable and highly anomalous gorilla... The gorilla was miniscule, but huge compared to the nodules. It was about the size of a box of matches-or 48 times the size of a typical nodule.

After they were done scrolling through the images as much as they wanted, the scientists asked them: Did that last trial seem any different? Did you notice anything unusual on the final trial? And finally: Did you see a gorilla on the final trial? Twenty of the 24 radiologists failed to see the gorilla, despite scrolling past it more than four times on average. And this was not because it was difficult to see: When shown the image again after the experiment, all of them saw the gorilla. What's more, the eye-tracking data showed clearly that most of those who did not see the gorilla did in fact look right at it.

What can we learn from the experiment? Well, the point isn't to admonish radiologists. Rather, the test was designed to investigate whether being highly trained made people less susceptible to the phenomenon of change blindness. Clearly, it doesn't.

It is, however, hard to get round the fact that 83 per cent of highly trained physicians missed what could've been a life-threatening anomaly. If that had been a tumour not a gorilla, it may never have been spotted — and that's certainly something worth worrying about. [Psychological Science]